Cyanobacteria have been a problem before now round the Wellington Region [my emphasis]:
In the summer of 2005/06, thick mats of cyanobacteria were found in some reaches of the Hutt, Mangaroa, Wainuiomata, Otaki, Waikanae and Waipoua rivers. The Hutt River was affected for much of the summer, with extensive thick, dark-brown/ black mats of Phormidium sp. present on the river margins in the Boulcott-Avalon area during a period of extended low river flows in November 2005. At least five dogs died around this time, after coming into contact with the algae at the water?s edge. Analytical tests confirmed the presence of toxins, leading Regional Public Health and local councils to erect health warning signs restricting access to affected rivers in the region over the summer.
Now the problem’s flaring up again and your dogs could be at risk:
River warnings (Updated 27 November 2012)
There is currently a moderate risk to river users from potentially toxic cyanobacteria in the Hutt River at Birchville, Maoribank Corner and Silverstream. Moderate growth of cyanobacteria mats has been recorded in these areas and detached mats are starting to wash up on the rivers edge. River users, particularly those with dogs, should avoid contact with algal mats in these river reaches. …
Rivers users, particularly those with dogs, or those drawing water from the river for stock/human consumption, should be on the lookout for exposed or easily accessible algal mats (usually brown or black in colour) both on the riverbanks and/or ‘floating’ on the surface of shallow areas and rocks in the rivers, and avoid any contact with them.